Chemically recyclable alternating copolymers with low polydispersity from conjugated/aromatic aldehydes and vinyl ethers: selective degradation to another monomer at ambient temperature

Abstract

A highly efficient chemical recycling system, involving repetitive cycles of precision synthesis and selective degradation of product copolymers, was developed based on controlled cationic alternating copolymerization of conjugated/aromatic aldehydes, such as cinnamaldehyde (CinA) and (E,E)-5-phenylpenta-(2,4)-dienal (PPDE), via exclusive 1,2-carbonyl addition with vinyl ethers (VEs).

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An effective approach to establishing a sustainable polymer production system is the synthesis of a polymer that can be selectively degraded to its original monomers under relatively mild conditions, which is termed as “tertiary recycling”. The tertiary recycling is exemplified by poly(lactide),¹ polyhydroxybutyrate,¹ poly(glycolide derivatives),^1,2 polyacetals,^3,4 polycarbonates,⁵ polyurethanes,⁶ and poly(cyclic orthoesters).^7–11 In contrast to these polymers, which are typically prepared by polycondensation, polyaddition, or ring opening polymerization, polymers of vinyl monomers would require much more severe conditions for complete degradation to their monomers because C–C bonds must be cleaved. Thus, a practical cyclic monomer–polymer recycling system has not been developed for polymers prepared by vinyl-addition polymerization, although highly effective production of α,ω-bifunctionalized oligomers from polyolefins were studied under appropriate degradation conditions toward recyclable polymers.¹

A degradable main chain would be obtained even from addition polymerization if a monomer with a carbon-heteroatom unsaturated bond, such as an aldehyde, was utilized. If the degradation product from the resulting degradable polymer is polymerizable, an endless synthesis–degradation cycle may be established. Recently, our report demonstrated that well-defined degradable copolymers were obtained from controlled alternating cationic copolymerization of benzaldehyde derivatives (BzAs) with vinyl ethers (VEs).¹² Since the VE–aldehyde sequence makes a stable but acid-labile acetal linkage, the copolymers can be readily decomposed by acid hydrolysis. In addition, the degradation of the copolymers yielded cinnamaldehyde (CinA) derivatives as nearly single hydrolysis products.

Because the degradation product is a CinA derivative, which is a potential monomer in cationic polymerization, the copolymerization system described here is a novel chemical recycling system: copolymerization, followed by complete and selective degradation with production of another monomer that can be further copolymerized with a VE. This type of recyclable polymer that provides another monomer as a degradation product has hardly been reported before. However, CinA derivatives have never been examined as monomers in chain-growth polymerization or in cationic polymerization. Only a few examples have been reported of their use in polymer science as initiators,^13,14 monomer precursors,¹⁵ and monomers for step-growth polymerization.^16–24

An essential requirement for realizing a repeating recycling system is successful alternating copolymerization of CinA proceeding via exclusive 1,2-carbonyl addition. Other addition modes will hinder degradation or lead to multiple degradation products. Aldehydes with a further extended conjugation system linked to the benzene ring would polymerize through more complicated addition modes (1,4-, 1,6-, 3,4-, etc.). Another requirement is selective degradation of product copolymers into another conjugated aldehyde with more extended conjugated double bonds that is also polymerizable.

This report describes a new type of chemical recycling system involving controlled alternating copolymerization and selective acid hydrolysis of product copolymers (Scheme 1). The alternating copolymerization of CinA with VEs via specific 1,2-carbonyl addition of the enal fragment was accomplished successfully, followed by the selective production of another conjugated aldehyde, (E,E)-5-phenylpenta-(2,4)-dienal (PPDE), through acid hydrolysis of the product copolymers. A subsequent cycle of alternating copolymerization and selective acid hydrolysis also was achieved using PPDE and isobutyl VE (IBVE) as co-monomers.

Scheme 1 A novel chemical recycle system consisting of alternating copolymerization of conjugated/aromatic aldehydes with VEs and selective acid hydrolysis of product copolymers.

The cationic copolymerization of CinA with IBVE was performed using the EtSO₃H–GaCl₃ initiating system, the most effective initiator/catalyst combination for copolymerization reactions of VEs with BzAs or naturally occurring conjugated aldehydes,^12,25,26 in the presence of 1,4-dioxane as an added base in toluene at −78 °C (Fig. 1). The reaction was almost complete in 4 h achieving 95% and 92% monomer conversion of IBVE and CinA, respectively (Fig. S1A†). The M_ns of the copolymers produced increased in direct proportion to CinA conversion, and their molecular weight distributions (MWDs) were narrow (M_w/M_n = 1.15–1.18) (Fig. 1A). In addition, the MWD curves of the polymers clearly shifted toward the higher MW region, indicating that long-lived species formed (Fig. S1B†). The amount of cyclic trimer^12,25,26 produced in the copolymerization was very low (≤5%) (Fig. S1B†).

Fig. 1 Cationic alternating copolymerization of CinA or PPDE with IBVE and acid hydrolysis of product copolymers: (A) M_n and M_w/M_n for polymer peaks of products obtained by copolymerization of CinA or PPDE with IBVE ([CinA]₀ = 0.60 M or [PPDE]₀ = 0.45 M, [IBVE]₀ = 0.60 M for CinA or 0.45 M for PPDE, [EtSO₃H]₀ = 4.0 mM, [GaCl₃]₀ = 4.0 mM, [1,4-dioxane] = 1.0 M, [DTBP] = 4.0 mM for PPDE, in toluene at −78 °C), and MWD curves of original polymers (upper) and hydrolysis products (lower) for (B) poly(CinA-co-IBVE) and (C) poly(PPDE-co-IBVE) [hydrolysis conditions: 0.50 M aqueous HCl–THF at 30 °C for 2 h for (B) and 1.0 M aqueous HCl–THF at 0 °C for 48 h for (C), 0.33 wt% polymer solution; aldehyde contents of original copolymers: (B) 48% and (C) 49%].

On the other hand, homopolymerization of CinA never proceeded under similar reaction conditions ([CinA]₀ = 0.60 M, [EtSO₃H]₀ = 4.0 mM, [GaCl₃]₀ = 4.0 mM, [1,4-dioxane] = 1.0 M, in toluene at −78 °C). The non-homopolymerizability would mean that CinA–CinA sequences are never formed and hence relative reactivities between a VE-derived cation and a VE or CinA determine polymer sequences from homopolymers of a VE to alternating copolymers in their copolymerization reactions.

Microstructures of the product copolymers were analyzed by ¹³C NMR after removal of the cyclic oligomers by reprecipitation. As shown in Fig. 2A, resonance peaks of aliphatic carbons derived from the IBVE units and aromatic carbons of the CinA units were observed, suggesting that each monomer was polymerized. The absence of resonance peaks attributable to carbonyl carbons indicated that no vinyl addition reaction of CinA occurred. In addition, resonance peaks of olefinic carbons connected to oxygen were not detected in the range of 140–160 ppm, which provides evidence that the 1,4-carbonyl addition reaction did not occur. These results and the peaks assignable to acetal carbons at ca. 100 ppm revealed that exclusive 1,2-carbonyl addition polymerization of CinA proceeded, similar to non-aromatic, conjugated aldehydes.²⁵ Although CinA is a phenyl-substituted enal compound and has an electronic nature different from that of alkyl-substituted enals, CinA reacted with the propagating cationic species in a manner similar to non-aromatic conjugated aldehydes. This suggests that the carbocation adjacent to an oxygen atom (–O–C⁺–C=C–Ph) was involved in resonance structures with an olefinic moiety likely playing a critical role in the controlled polymerization. Similar selective 1,2-addition reactions of CinA with nucleophiles were also reported in Prins reactions: CinA activated by a proton or Lewis acid was reacted with a vinyl ether moiety²⁷ or allylic compounds^28,29 to produce each product via 1,2-carbonyl addition reaction.

Fig. 2 (A) ¹³C and (B) ¹H NMR spectra of poly(CinA-co-IBVE) [M_n(GPC) = 2.14 × 10⁴, M_w/M_n(GPC) = 1.15, aldehyde content: 48%]; and (C) ¹³C and (D) ¹H NMR spectra of poly(PPDE-co-IBVE) [M_n(GPC) = 1.68 × 10⁴, M_w/M_n(GPC) = 1.20, aldehyde content: 49%] [125.77 MHz (for ¹³C NMR) and 500.16 MHz (for ¹H NMR), in CDCl₃ at 30 °C].

The CinA content of each copolymer was determined using ¹H NMR integral peak ratios (Fig. 2B). The contents, measured by the ratio of the methine peaks (b and f) and the methyl peak (e), were calculated to be 46–48%. These values, the lack of CinA homopolymerizability, and the exclusive 1,2-carbonyl addition reaction strongly supported the structure of the alternating copolymers of CinA and IBVE shown in Fig. 2. No reaction of the olefin bonds in the side chain was confirmed by the nearly constant peak integral ratios of the olefinic protons (g and h) and the methine peaks (b and f) through the copolymerization.

Acid hydrolysis of the alternating poly(CinA-co-IBVE) was conducted after the removal of cyclic trimer byproducts through reprecipitation. Fig. 1B shows the GPC traces of the original polymers and hydrolysis products. The original alternating copolymers were quantitatively degraded by acid hydrolysis and a nearly single product was confirmed. The NMR spectra of the hydrolysis products were subsequently recorded to identify the products. Intense resonance peaks in the ¹H NMR and ¹³C NMR spectra of the product revealed that PPDE, a conjugated aldehyde shown in Fig. 1B, was a selective hydrolysis product (Fig. S2 and S3†).³⁰ Exclusive production of PPDE, a phenyl dienal compound with π-conjugation that extends two carbons beyond that of CinA, indicated that hydrolysis proceeded via the mechanism discussed in a previous report and that the copolymer had an alternating sequence resulting from a 1,2-carbonyl addition reaction.¹²

Accordingly, the well-controlled alternating copolymerization of CinA with IBVE and subsequent selective degradation of the product copolymers into PPDE were achieved as expected. Therefore, a new type of chemical recycling system was established up to the second cycle (Scheme 1).

Toward the third cycle, PPDE, the selective hydrolysis product, was copolymerized with IBVE (Scheme 1; [PPDE]₀ = 0.45 M, [IBVE]₀ = 0.45 M, [EtSO₃H]₀ = 4.0 mM, [GaCl₃]₀ = 4.0 mM, [1,4-dioxane] = 1.0 M, in toluene at −78 °C: Fig. S4†). PPDE also functioned successfully as an effective co-monomer in the copolymerization, which smoothly produced high monomer conversions (86% for IBVE and 83% for PPDE in 8 h; Fig. S4A†).³¹ In addition, long-lived growing species were observed in the GPC traces of the products (Fig. S4C†). The M_ns of the product copolymers increased in direct proportion to PPDE conversion to the polymer (M_n = 3.7–13.6 × 10³), and the MWDs were relatively narrow (M_w/M_n = 1.24–1.37) (Fig. S4B†). In the later stage, however, the M_n deviated from the straight line, probably due to transfer reactions. To suppress the side reactions, the copolymerization was re-examined in the presence of the proton trap reagent 2,6-di-tert-butylpyridine (DTBP).³² Results of the copolymerization are shown in Fig. 1A and S5.† A longer reaction time was required for high monomer conversions compared to the reaction without DTBP (85% for IBVE and 82% for PPDE in 48 h; Fig. S5A†), most likely due to a decrease in the concentration of GaCl₃ as a result of the formation of (H–DTBP)⁺(X–GaCl₃)⁻ salts through scavenging of protic impurities and/or eliminated protons,³³ and/or the direct interaction between GaCl₃ and DTBP. The controllability of the copolymerization was improved and produced a linear relationship between the M_ns of the product copolymers and the PPDE conversions throughout the reaction (Fig. 1A). Therefore, controlled copolymerization of PPDE and IBVE was demonstrated under appropriate reaction conditions.

PPDE is a dienal compound, i.e. an aldehyde having a diene structure, that can undergo multiple and complex addition reactions. Despite the increased possibility of multiple addition modes, again, only 1,2-carbonyl addition occurred, producing copolymers with simple repeating units, as observed in the copolymerization reactions of other conjugated aldehydes. This structure was supported by ¹³C NMR (Fig. 2C), which did not show a resonance peak from carbonyl carbons, evidence for no addition occurring through the diene moiety. The ¹³C NMR spectrum also did not contain a resonance peak derived from oxygen-connected olefin carbons in the range of 140–170 ppm, indicating the absence of 1,4- or 1,6-carbonyl addition reactions. Thus, the exclusive 1,2-carbonyl addition reaction must have occurred even when the dienal reactant was used.

Fig. 2D shows the ¹H NMR spectrum of the product copolymer. Because the resonance peaks of aldehyde protons were not present, addition reactions of the IBVE growing ends with a diene fragment of PPDE could again be excluded. The PPDE contents of the product copolymers were determined based on the integral ratio of resonance peaks (b and f) and (e). The nearly alternating fashion of the copolymers was indicated by the PPDE contents (47–49%).

As described above, NMR studies of the product polymers obtained from copolymerization of PPDE with IBVE revealed that alternating copolymerization preferentially based on 1,2-carbonyl addition of the dienal moiety of PPDE was achieved. The addition reactions of growing carbocations with the phenyldiene skeleton in the polymer side chains also were negligible because high MW portions resulting from interpolymer reactions were not detected by GPC (Fig. S5B†).

The resulting alternating copolymer, poly(PPDE-co-IBVE), was hydrolyzed under conditions similar to those for the hydrolysis of poly(CinA-co-IBVE) at 0 °C.³⁴ This reaction provided a high yield of (E,E,E)-7-phenylhepta-(2,4,6)-trienal (PHTE) (∼80%). The ¹³C NMR spectrum of the hydrolysis product exhibited intense resonance peaks of PHTE (Fig. S6†),³⁰ although small broad resonance peaks also appeared at 126–130 ppm, most likely due to aromatic and/or olefinic carbons of the side products from PHTE itself (Fig. S6†).³⁴ As a result, a highly efficient hydrolysis reaction of alternating poly(PPDE-co-IBVE) to produce PHTE was achieved. Thus, the third cycle of alternating copolymerization and selective acid hydrolysis of conjugated/aromatic aldehydes with VEs was almost established (Scheme 1).

In conclusion, chemically recyclable polymers consisting of conjugated/aromatic aldehydes and VEs were developed. The chemical recycle began with well-controlled cationic alternating copolymerization of CinA with VEs via 1,2-carbonyl addition only as the second cycle, which was demonstrated successfully for the first time. The alternating copolymers that resulted were transformed quantitatively into PPDE by acid hydrolysis.³⁵ PPDE is a conjugated aldehyde with a structure elongated by two carbons compared to CinA. Interestingly, PPDE and IBVE also could be copolymerized via 1,2-carbonyl addition polymerization to produce their alternating copolymers in a controlled manner in the presence of DTBP as a proton trap. The hydrolysis of the alternating copolymers yielded PHTE, the phenyl-substituted enal compound with a two-carbon inserted conjugated structure from PPDE, in high yield.³⁵ Thus, a novel chemically recyclable process for synthesizing polymers consisting of conjugated/aromatic aldehydes and VEs was demonstrated. Obtained alternating copolymers in the second and third recycling steps possessed thermal properties similar to those of the first generation³⁶ and would be industrially utilizable as photoresists or temporary adhesive materials. In addition, the reaction pattern itself, i.e., exclusive 1,2-addition reactions of conjugated aldehydes with the benzene ring, is unprecedented. This recycling system has potential as a new synthetic method for conjugated/aromatic aldehydes. Furthermore, functionalization of alternatingly arranged olefinic moieties would be intriguing, giving unique polymers with specific properties as well as selective degradability.

This research was supported in part by a Grant-in Aid for Scientific Research (no. 22107006) on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (no. 2206), and a Grant-in-Aid for JSPS Fellows (no. 23-1924) for Y. Ishido from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). Y. Ishido thanks The JSPS Research Fellowships for Young Scientists and The Global COE Program “Global Education and Research Center for Bio-Environmental Chemistry” of Osaka University. We thank Prof. T. Inoue group (Osaka University) for DSC experiments and Prof. K. Imada group (Osaka University), especially Associate Professor Fumitoshi Kaneko, for TGA experiments.

Notes and references

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34. To minimize side reactions of PHTE itself, acid hydrolysis was performed at 0 °C. GPC traces of the hydrolysis products obtained at 30 °C after 2 h reaction showed not only an intense RI peak derived from the conjugated aldehyde PHTE, but also weak oligomer peaks (M_w = 200–2200). When the alternating copolymers were allowed to remain under acidic conditions for an extended period (24 h), oligomer amounts increased slightly rather than decreased. The NMR spectra of these products revealed that no acetal linkages remained in the oligomers. Therefore, the oligomers could not be undegraded polymer fragments, but must be reaction products from the conjugated aldehydes, i.e., the selective hydrolysis product PHTE, which would possess high reactivity due to its larger π-conjugated system.
35. To reveal the hydrolysis mechanisms, a degradation reaction of the alternating copolymer from a VE with a BzA was examined (Fig. S7†). The hydrolysis reaction of poly(p-methoxyBzA-co-IBVE) [M_n(GPC) = 2.70 × 10⁴, M_w/M_n(GPC) = 1.25, p-methoxyBzA content: 45%] was quenched in a short time (1 min). The GPC peak of the hydrolysis product [M_n(GPC) = 4.30 × 10³, M_w/M_n(GPC) = 1.75] obviously shifted toward the lower MW region compared to the virgin copolymer. However, the MWD was unimodal without any intense peaks in the MW range of 100–200 (Fig. S7†), which suggests that no or very few CinAs were produced in the early stage of the hydrolysis reaction. These results indicated that the degradation of the alternating copolymers did not proceed via unzipping type reactions but by random scissions of the main chain.
36. Glass transition temperature (T_g) and decomposition temperatures, the temperature of 5% weight loss (T_5%) of samples, were measured. Poly(p-methoxyBzA-co-IBVE) [M_n(GPC) = 1.82 × 10⁴, M_w/M_n(GPC) = 1.09, p-methoxyBzA content: 45%], a first generation polymer, exhibited T_g and T_5% of 35 °C and 315 °C, respectively. Poly(CinA-co-IBVE) [M_n(GPC) = 1.80 × 104, M_w/M_n(GPC) = 1.16, CinA content: 48%], a second generation polymer, had T_g and T_5% of 30 °C and 289 °C, respectively.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and figures showing time-conversion curves, M_n and M_w/M_n-conversion plots of copolymerization, MWD curves and NMR spectra. See DOI: 10.1039/c3py00842h

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